Data can be communicated over electrical wires or optical fibers in a communications network. The electrical or fiber data may be formatted and defined according to any number of varying protocols and varying standards, depending on the application in which the data is used. In addition, these protocols and standards require different transmission rates, and the data may be synchronous, plesiosynchronous, or asynchronous. Organizing all of these so-called tributaries of data into a single data stream for transmission over an optical fiber requires mapping (or de-mapping) these tributaries into a frame of a well-known hierarchy called the synchronous optical network (SONET), the standard for such communications in North America, or synchronous digital hierarchy (SDH), SONET's European counterpart.
The basic building block of a SONET frame is a synchronous transport signal, level 1 (STS-1), which has a transmission rate of 51.84 Mbps. Higher transmission rates are supported by combining (or concatenating) multiple STS-1 signals together. Transmission rates below the STS-1 level are also supported. The synchronous payload envelope (SPE) of an STS-1 is subdivided into virtual tributaries (VT). The smallest VT, VT1.5, has a transmission rate of 1.728 Mbps. Higher bandwidth VTs are defined and include VT2, VT3, and VT6. Like the STS-1 signals, VT signals can be concatenated together to support different transmission rates. Using these basic building blocks of STS-1 and VT1.5, a variety of different data signals can be communicated over an optical fiber.
Mapping and de-mapping these data signals between an optical fiber line and client interfaces are typically carried out using a framer/mapper device. The device extracts information in the SONET frame to locate the data carried within the frame, typically by finding two bytes called the A1 and A2 bytes in the SONET frame, which indicate the start of the SONET frame. This device terminates SONET transport overhead with respect to client interfaces, and it converts the client interface signals between their respective protocol formats and SONET/SDH format. A tributary connected to the device also receives all of the data in the SONET frame, and the tributary must perform its own SONET framing and mapping to map and de-map its data relative to the SONET frame. These redundant functions performed by the tributary increase hardware and software complexity. The tributary must also know the bandwidth of the data communicated between it and the framer/mapper device.
Moreover, there is no centralized assignment of payload data relative to the various tributaries, raising the possibility of contention and security issues. For example, because each tributary is unaware of what other tributaries are adding to and dropping from the bus, multiple tributaries may attempt to map payload data into the same portion of the SONET frame. These issues can be partly addressed, but at a cost of increasing hardware and software complexity.
What is needed, therefore, is a flexible and expandable system bus for facilitating the mapping and de-mapping of varying communications protocols relative to a SONET/SDH frame. The present invention addresses this and other needs.